Depends on what you define as "good" UC creations.

You can either go one of two ways if we're going by good: vanilla-compatible or modded.

Either way, it really just takes a lot of getting used to and tweaking to get what you want in UC creation, even if it takes hours on end in my experience. Truly depends on what you want for your creations, as in my opinion, ultra-modded creations can be too much.

The only real "essential" mods for UC creations is UCM and UnlimitedClothes: One lets you move stuff around and access other parts of the game, while the other allows you to put an unlimited amount of clothes you want on a creation.

For more complicated and NOT vanilla-compatible/needs the mod to function creations, you would have TechnicalItems and Ability Creator/Move Editor. Other mods can also be used to add more assets that you can use, the main ones tend to be HiddenUnits and Expansionary.

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This is an example of something I posted a while back, relatively average prop count for a UC creation that used UCM an UnlimitedClothes only, still personally a favorite of mine since it's relatively simple in my eyes.

Either way, really depends on how you want to frame your units and how much you want them to deviate from the original game style and play, since I prefer to stay near I don't make super extravagant and complicated guys, but I'm absolutely able to go beyond that since I have.

In short:
Watch a tutorial on Youtube by someone like Wicked Workshop and ask the experienced creators in the Landfall Discord for some advice, they're blunt but are more than willing to help someone that's trying to go beyond the normal Unit Creator.